关闭背景时亮度可调

新增自定义图片背景功能（可调透明度，目前仅电脑可用）

parts/backgrounds/gray.lua

@@ -0,0 +1,11 @@
+--Customizable grey background
+local gc=love.graphics
+local back={}
+local brightness=.26
+function back.draw()
+    gc.clear(brightness,brightness,brightness)
+end
+function back.event(b)
+    brightness=b
+end
+return back